The present invention is generally related to metallic articles and, more particularly, is related to a method for forming metallic articles from non-metallic articles.
The process of producing metals by direct reduction of non-metal oxides is well known in the art.
Conversion to Metal by Direct Reduction:
The direct reduction (DR) process produces metal directly from metal-bearing ores or oxides by removing the associated oxygen or other anions at temperatures below the melting temperature of any of the materials involved in the process. Iron, for example, has been produced in this manner prior to the invention of blast furnace, in which iron is melted and reduced with carbon and withdrawn as a liquid, molten metal. Direct-reduced iron (DRI) is normally produced in the form of lumps or agglomerates due to extremely high volume shrinkage (53.4%) and weight loss (30%). Numerous phase changes are also present in the course of reaction, which causes additional movement of atoms in the substance. Whether the product maintains its original shape or its structural integrity has not been a concern to any degree because DRI is solely used as substitution for steel scrape to boost production rate of other steel-making processes in either a blast furnace or an electric arc furnace.
Direct reduction of a metal oxide requires energy in the presence of suitable reducing agents. Common reductants for iron as an example include, but are not limited to hydrogen, carbon monoxide, methane, coal gas, fuel oils, coal, coke, etc. The reducing agent reacts with oxygen and forms water molecules, which are then removed from the system. It is a well known that the reaction rate will increase as the concentration of reactants increases and decrease as the concentration of reaction products increases (Le Chatelier""s Principle). The reaction rate is a strong function of the available concentration of reactants, both the metal oxide and reducing agent; the concentration of reaction products (water vapor); and temperature.
Metals can also be produced from other non-metallic metal precursor materials such as hydrides. In this case, metal hydrides such as titanium hydride can be chemically converted to the metal by heating the metal hydride to a high enough temperature to decompose the hydride. For titanium hydride the decomposition temperature is approximately 600xc2x0 C. Above the decomposition temperature, the titanium hydride separates into titanium and hydrogen gas. At higher temperatures, the titanium powder can be consolidated to a dense metal by solid state sintering. This process also applies to hydrides of vanadium and zirconium.
Shape Fabrication:
Fabricating nonmetallic articles with a specific geometry from nonmetallic metal precursors is well known in the art. Various methods of shape fabrication of nonmetallic articles are outlined below.
a. Dry Pressing
The most common method that can be used to consolidate powders as starting materials into a useful shape is xe2x80x9cdry pressing,xe2x80x9d which is a traditional forming process. Although the name includes xe2x80x9cdryxe2x80x9d as a modifier, the starting materials usually contain a few percent by weight of moisture to differentiate from wet or semi-wet pressing, such as the xe2x80x9cstiff mudxe2x80x9d process. The basic dry pressing process involves applying the pressure uniaxially. If pressure is applied from all directions, or isostatically, then the process is called xe2x80x9cisostatic pressing.xe2x80x9d Whether the pressure is to be applied uniaxially, biaxially or isostatically, the decision is largely dependent upon the property requirements and manufacturing economics.
Uniaxial pressure fabrication is a very common forming process. It is used to form many tiles and other flat shapes, as well as simple shapes such as disks or cylinders. The cross-sections that can be formed are usually fairly simple geometrically, although the pressed shapes can be machined into more complex geometries. A large number of holes can be made to the pressed parts with the aid of inserts. The height is usually limited relative to the lateral dimension or diameter. Pressing of floor tile is one example of a dry pressing process that has been highly automated.
b. Slip Casting:
Conventional slip casting is a process for forming articles with a suspension of ceramic powders. Water is usually used as the liquid medium although some nonaquous solvents have been used in certain situations.
c. Pressure Casting
Pressure slip casting is basically the same process but with pressure applied to the slip in the mold. Pressure casting is being applied in the sanitary ware industry and has produced a number of advantages. Casting times are significantly cut and parts can be easily demolded. Molds usually require no drying between casting cycles and thus can be returned to service immediately, as an air purging system is used to de-water molds. Mold life is much longer than conventional plaster molds, and fewer defects occur because of mold wear. Moreover, product quality is more consistent and the cast part has less moisture to remove. This eases drying requirements and cuts drying defects and losses. Parts with variable thickness are easier to mold. One person can operate two or three casting machines, including fettling of parts, and the operation can be run two or three shifts per day. The net result is greater throughput, lower labor costs, and lower overall production costs.
d. Centrifugal Casting:
Centrifugal slip casting is another means of increasing pressure at the casting face, but with lower pressure than in the pressure casting process.
e. Gel Casting:
Gel casting is a recently developed technique for water and is currently being used to form complex shape ceramic rotors for automotive turbochargers.
f. Slurry:
There are multiple casting slip properties that are desirable to allow an optimum process. These properties include: 1) low viscosity, i.e., high flow rates, to allow all parts of the mold to be easily filled and to prevent trapping of air bubbles; 2) high specific gravity to shorten casting time, increase green density, lower drying shrinkage, and lower the amount of water that must be processed; 3) good casting rate; 4) easy mold release; 5) adequate draining behavior from the mold at the end of the cast; and 6) sufficient green strength in the cast layer to allow ease of handling.
g. Extrusion
Extrusion can be a very effective and efficient method of forming material continuously or semi-continuously using relatively simple equipment. The advantages of extrusion as a forming and consolidation process have been recognized and utilized by manufacturers of nearly all materials. If a material can be melted, softened, or mixed into a plastic state so that it can be forced through a die, then it can be and probably has been extruded.
In terms of material and energy conservation, the net shape and continuous forming capabilities of the extrusion process are very attractive. Extrusion has been used for many years in the clay/porcelain industries. More recently, it has been used with fine, technical ceramics such as silicon carbide, silicon nitride, and oxide materials. Shape capability has also expanded greatly, from simple rods and tubes to complex profiles, sheets/films, and honeycombs.
Extrusion has limitations and cannot be used to make all products. It is best suited to fabricate shapes that are of a constant cross section and can be linearly formed. Typical products formed by extrusion are: tubes or pipes, with either open or closed ends; profiles of numerous shapes; rods; honeycombs; plates (solid, hollow, or ribbed); and films.
It has been heretofore unknown how to use the above prior art shape fabrication methods to achieve metallic articles. Further, due to problems with controlling the reduction reactions, the method of producing metallic articles by direct reduction has been limited in the prior art to basic forms, i.e., flat sheets. What is desired, but has been heretofore unaddressed in the prior art is a method of producing metallic articles of intricate or varied geometrical shapes that possess high transverse strength.
The present invention provides a method for producing metallic articles by direct reduction of metallic oxide articles or decomposition of metal hydride articles. Briefly described, the process for forming a shaped metallic article includes the steps of combining starting materials, which include non-metallic metal precursor powder(s), a binder, and a solvent, forming the starting materials into a shape to produce a nonmetallic article of a certain geometry, and converting the nonmetallic article to a metallic article with a reducing agent or thermal decomposition., while substantially retaining the geometry of the nonmetallic article.
The present invention can also be viewed as providing a method for producing a shaped metallic article wherein the forming step includes extruding the non-metallic metal precursor powder article into a shape. Further described, the extrusion process includes the steps of forming the starting materials into a paste, extruding the paste into a non-metallic metal precrusor powderarticle with a certain geometry, drying the metal precrusor article, removing the binder from the nonmetallc article, reducing and/or decomposing the nonmetallic article to a metallic article while substantially retaining the geometry of the metallic oxide article, sintering the metallic article to form a more dense metallic article, and heat treating the dense metal shape.
The present invention can also be viewed as providing a method for producing a shaped metallic article wherein the shape fabrication process includes dry pressing of the metal precursor article. In this regard, the method can be broadly summarized by the following steps: spray-drying the starting materials to form a pressing powder, dry-pressing the pressing powder, delivering the pressing powder into a die cavity to form a nonmetallic article, and then subjecting the nonmetallic article to a direct reduction process.
The present invention can further be viewed as providing a method for forming and nonmetallic article by a slurry forming process. In this regard, the slurry forming process can be broadly summarized by the following steps: forming a slurry of the starting materials, mixing the slurry, casting the slurry to produce a nonmetallic article with a certain geometry, and then reducing the nonmetallic article to a metallic article with substantially the same geometry as the nonmetallic article.